1. Field of the Invention
The present invention relates to the field of fasteners and nails with integral locking means comprising protrusions on the shank in the form of longitudinal ribs.
2. Related Art
Nails have been around for thousands of years. During that time they have been made using different methods. One-way of forming a nail was to start with a bit of steel and shape it by hand on an anvil. Another way was to cut elongate rectangles from a sheet of steel. During these earlier times, nails were a high tech item and were quite valuable. It was not uncommon to burn down old buildings in order to collect the nails from the ashes.
In the early 1800s there was a change in the way nails were made. Wire making had been developed to a point that wire was readily available and round shaped nails, made from wire, began to appear. These nails were made from a length of wire that had a flattened cylindrical head hammered on one end and a point cut on the other. This form of nail was very easy to make from wire and has not changed in any fundamental way for over one hundred years. Some minor features such as enlarged heads, cement coatings, or surface textures have been added in order to improve the holding power when used is specific applications such as dry wall or roofing operations.
There are some disadvantages to a round cylindrically-shaped nail.
First, a round cylindrical nail uses the maximum amount of steel for a given cross-section. The cost of the steel wire is between 50 and 80 percent of the total expense in making nails, and any decrease in the steel used in each nail would lead to a significant increase in potential profit.
The second disadvantage of a round cylindrical nail is that it has a tendency to wedge wood apart. Assuming the wood is not split, a common occurrence, the wood fibers spread apart in such a way as to form an eye-shaped opening around the nail. This shape decreases the holding power of a round nail by reducing the amount of surface area, the nail perimeter times its length, which is in contact with the wood. As a corollary to the first disadvantage, a round nail has the smallest amount of surface area for any amount of steel.
The disadvantages of a round nail have not gone unnoticed. U.S. Pat. No. 340,692, issued Apr. 27, 1886 to Bailey for a “Wire Nail,” discloses a nail with a dozen or so longitudinal ridges and channels on the shank. In the illustrations, the height of the ridges and depths of the channels are not large, but the nail is useful as a furniture nail because it will not turn on its axis. Also, the strength to weight ratio is increased.
Rail spikes, rather than join two pieces of wood, hold down an iron rail. The traditional rail spike has an almost square cross-section. However, there have been a large number of attempted improvements in this field as well. An example is U.S. Pat. No. 927,301, issued Jul. 6, 1909 to Walcott for a “Spike”. This patent discloses a modified rail spike with a shank that has a half-round shape opposite the rail side and, on the rail side, three longitudinal convex flanges with two intervening concave grooves. The inventor emphasizes that “The curves of the concave grooves run smoothly and continuously into the convex flanges without angles.” This has special value in avoiding splitting of the tie and increasing holding power.
U.S. Pat. No. 2,014,746, issued Sep. 17, 1935 to Robergel for “Method of Making Nails,” discloses a nail having a “cross-shaped or, more generally, star shaped” shank. FIG. 3 illustrates the cross-shaped version as having rectangular arms of equal length. The method of making this particular part of the nail is not specified, but it is pointed out that the beginning and ending cross-section have the same area.
U.S. Pat. No. 4,637,768, issued Jan. 20, 1987 to Rabe for a “Nail-Type Fastener . . . ,” discloses a nail having a shank cross-section with three four-sided V-like grooves disposed at 120° around the nail shank. This cross-section is manufactured with three cold-rollers at 120° followed by a second set to deepen the grooves. The purpose of the grooves is to increase the ratio of frictional surface to weight so as to increase pull grip. The ribs between grooves increase flexural rigidity. The cold-rolling operation is designed to produce material flow in both longitudinal and transverse directions. The transverse flow is to increase strength.
U.S. Pat. Nos. 4,755,091 issued Jul. 5, 1988 to Potucek et al. for “Star Fasteners” and 4,973,211 issued Nov. 27, 1990 to Potucek for “Star Fastener,” disclose non-round nail shapes. The preferred embodiment appears to be a nail with a five pointed star-like cross-section. These nails performed as well as, or in some cases better than traditional nails and were made using 50% less steel. These patents also discuss a 15% increase in surface area compared to a round nail of the same overall diameter and a 60% increase in holding power due to wedging and compression of fibers in the cups between the star points. As illustrated in U.S. Pat. Nos. 4,800,746, issued Jan. 31, 1989 and 4,833,906, issued May 30, 1989, both to Potucek, five pointed nails could be produced by cold rolling five grooves longitudinally down round wire feedstock.
U.S. Pat. No. 5,836,826, issued Nov. 17, 1998 to Haraminac for a “Machine for Making Star Nails,” attempted to overcome some problems with the previous approaches by using form rollers, but on conventionally formed nails as opposed to wire bar feedstock. These grooves displaced 50% of the steel during the forming process, producing a nail with a finished outer diameter larger than the starting wire. Although the new shape appeared to be promising, attempts to manufacture nails of this shape for a commercial market have not yet been successful. There appear to be several reasons for this.
First, the rolling system required to produce this shape is not common to the nail industry. Beyond that, the star nail rolling system proved to be difficult and expensive to build and maintain. The rollers in test machines had rolling edges which would not hold their shape long enough to be economically viable, and the machines could not be made to operate fast enough to be competitive.
Second, the displacement of the wire during the rolling process was hard to control or predict. Displacing 50% of the material in order to achieve a 50% steel savings was a significant feature of the Potucek patents, but moving such a large amount of material created technical problems that have not been overcome to date.
The preferred embodiment in Potucek's Star Fastener(s) patents is quite clearly a nail shank having a circular central shaft with five radial fins (rather like a starfish with thinner arms than the animal). Although the number of fins can vary from three upwards, they are always radial. Even though the new star shape appears to be a significant improvement on a round nail, the very star shape may have hindered development of practical production machinery.
Lastly, U.S. Pat. No. 5,143,501, issued to Leistner et al. for a “Grooved Nail and Strip,” discloses three different nail shank cross-sections. One has four symmetrically disposed concave quarter circle grooves with radii the same or less than the half-width of the shank from ridge top to ridge top. Two opposing ridge tops are flat to facilitate forming a nail strip. Another cross-section has three similar, but larger radii, grooves and ridges at 120° and the third cross-section is essentially square. Compared to other non-circular cross sections, these are relatively smooth.
This patent teaches that “the holding power of a nail is simply the degree to which the wooden fibers are displaced transversely to, i.e., across the axis of the grain of wood,” see FIG. 5. Thus, it states that, in practice, teeth or notches and various forms of knurlings or serrations have produced very little, if any, increase in holding power. It goes on to state that, although “some manufacturers still insist on achieving the sharpest possible teeth or notches”, teeth or notches would tear the wood and reduce holding power. Therefore, the object of this invention is to provide increased wood separation for the same weight nail in a shape that also allows forming into nailing sticks. The cross-sections can be produced by extruding through a die with additional pointing and heading operations.
To add to the debate, one trick that experienced carpenters use to avoid splitting wood with common nails in some situations, is to blunt the point. This breaks some of the fibers so that it is easier to separate the remaining ones. Holding power should be reduced, however.
In spite of some inventive effort over many years, nails with non-circular cross-section shanks (or non-square ones in the case of rail spikes) as discussed in these patents are not in widespread use. Part of the problem is probably the difficulty in manufacturing. Therefore, what is needed is a nail shank design that, compared to a round one, provides reduced weight but has the same or increased holding power and can be manufactured using practical forming apparatus.